1. Field of the Invention
The present invention relates to a propeller for watercraft and an outboard motor.
2. Description of the Related Art
An outboard motor can be attached to a boat body by being simply engaged onto the stern of a boat, and does not occupy any space inside the boat. Therefore, outboard motors are widely used for small-sized boats, e.g., pleasure boats and small fishing boats. In accordance with the boat body sizes and purposes, outboard motors of various output powers are in use today.
Generally speaking, an outboard motor having a propeller made of stainless steel and an engine with high output power (e.g., 100 horsepower or more) is used for a relatively large boat. On the other hand, for a relatively small boat, an outboard motor having a propeller made of aluminum or the like and an engine with relatively low output power is used. An aluminum propeller is light-weight and can be produced at low cost, and therefore is suitable as a propeller of an outboard motor having an engine with a small output power.
In the case of forming a propeller for watercraft from an aluminum alloy, it is necessary to prevent corrosion of the aluminum alloy caused by seawater. Therefore, generally speaking, propellers having an aluminum alloy propeller body painted or coated with a corrosion resistant or preventive material are widely used.
Japanese Utility Model No. 3029215 discloses, in order to prevent deteriorations in water dissipation during the rotation of a propeller (which may happen when the propeller edge is made dull by any painted film that is provided on the propeller surface), subjecting an aluminum-alloy propeller to a hard anodized aluminum treatment is necessary to secure a sharp propeller edge.
Small-sized boats having an outboard motor are often used at inshore locations and on rivers, for purposes such as fishery, business operations, and leisure activities. Such boats may be pulled onto a sand beach for mooring, or may be moored in the sandy shallow area by a river shore. Therefore, when mooring a boat, or when going out onto the river or the sea from a point of mooring, sand may be stirred up, and the propeller surface is likely to be abraded as the propeller is rotated in the sand-containing water. As a result, the painting on the propeller surface may peel due to abrasion, the propeller body may be corroded, and the propeller body may be abraded. Since a paint coating does not have sufficient hardness, the propeller of a conventional outboard motor has a problem of short life ascribable to abrasion.
Japanese Utility Model No. 3029215 merely discloses forming an anodized aluminum layer (which is known as a corrosion-protective coating for aluminum), instead of a painted film for corrosion protection, without disclosing the aforementioned problems. Moreover, in order not to allow the propeller edge to become dull, it would be impossible to form a thick layer of hard anodized aluminum. Therefore, the thickness of the hard anodized aluminum layer for a propeller according to Japanese Utility Model No. 3029215 can only be about 15 μm, which is not considered to provide sufficient abrasion resistance.
Moreover, generally speaking, an aluminum-alloy propeller is molded by die casting or gravity casting. However, even a propeller after being molded is subjected to an anodic oxidation treatment as it is, variations may occur in the coating thickness. This makes it difficult to obtain sufficient abrasion resistance. Moreover, providing a thick coating in order to obtain sufficient abrasion resistance makes it necessary to perform anodic oxidation over a long time, which lowers the hardness of the film and hence invites a lower abrasion resistance.
Such problems occur not only in boats having outboard motors, but also in small-sized boats whose engines are mounted within the boats.